Fast sign of integer in C

Question

There is a sign function in C:

int sign(int x)
{
    if(x > 0) return 1;
    if(x < 0) return -1;
    return 0;
}

Unfortunately, comparison cost is very high, so I need to modify function in order reduce the number of comparisons.

I tried the following:

int sign(int x)
{
    int result;
    result = (-1)*(((unsigned int)x)>>31);

    if (x > 0) return 1;

    return result;
}

In this case I get only one comparison.

Is there any way to avoid comparisons at all?

EDIT possible duplicate does not give an answer for a question as all answers are C++, uses comparison (that I supposed to avoid) or does not return -1, +1, 0.

Answer 1

First of all, integer comparison is very cheap. It's branching that can be expensive (due to the risk of branch mispredictions).

I have benchmarked your function on a Sandy Bridge box using gcc 4.7.2, and it takes about 1.2ns per call.

The following is about 25% faster, at about 0.9ns per call:

int sign(int x) {
    return (x > 0) - (x < 0);
}

The machine code for the above is completely branchless:

_sign:
    xorl    %eax, %eax
    testl   %edi, %edi
    setg    %al
    shrl    $31, %edi
    subl    %edi, %eax
    ret

Two things are worth pointing out:

1. The base level of performance is very high.
2. Eliminating branching does improve performance here, but not dramatically.

Answer 2

int sign(int x)
{
    // assumes 32-bit int and 2s complement signed shifts work (implementation defined by C spec)
    return (x>>31) | ((unsigned)-x >> 31);
}

The first part (x>>32) gives you -1 for negative numbers and 0 for 0 or positive numbers. The second part gives you 1 if x > 0 or equal to INT_MIN, and 0 otherwise. Or gives you the right final answer.

There's also the canonical return (x > 0) - (x < 0);, but unfortunately most compilers will use branches to generate code for that, even though there are no visible branches. You can try to manually turn it into branchless code as:

int sign(int x)
{
    // assumes 32-bit int/unsigned
    return ((unsigned)-x >> 31) - ((unsigned)x >> 31);
}

which is arguably better than the above as it doesn't depend on implementation defined behavior, but has a subtle bug in that it will return 0 for INT_MIN.